In general, many electronic systems are today designed as modular systems composed of large numbers of a few types of general and special purpose plug-in circuits. Typically, the circuits are contained on circuit boards which are inserted into receptacles containing connectors which electrically engage terminals on the boards. For economic reasons, the different types of boards that might be required in any system are usually compatible with all the receptacles of the system. This means, of course, that without some form of constraint, any of the boards may be physically inserted into any of the compatible receptacles of the system, irrespective of whether or not it is the correct type of board for the receptacle. This, in turn, endangers the reliability of any such system due to human error when replacing faulty boards and the like.
In general, it is known to provide keying arrangements in connection with many types of plug-in devices to prevent inadvertent misuse. A common example is the use of different orientations of terminals and contacts in male and female power plugs and receptacles for different voltage and current ratings. The extension of this general principle of keying to circuit boards is also known.
For example, U.S. Pat. No. 3,177,461, which issued to T. G. Hagan on Apr. 6, 1965, discloses one circuit board keying arrangement in which tubular members are colinearly mounted in a receptacle and on a circuit board with their axes parallel to the direction of insertion of the board into the receptacle. Each tubular member has a portion of its tubular section removed as if a cut were made through its axis from one end thereof partially along its length and then at right angles out to its tubular surface. These tubular members are manually rotated to prescribed positions and locked in place to form the unique keys. Only when the ends of the tubular members in the receptacle and on the board intermesh can a board be completely inserted into the receptacle.
Another known circuit board keying arrangement involves placing slots in coded locations at the terminal end of a board. The slots run parallel to the direction of board insertion. Thin wires are located in front of and across the receptacle connector and align with the slots in a correct board or block the final insertion of an incorrect board into the connector.
The foregoing prior art and variations thereof, although effective, often suffer from one or more of several disadvantages. Some require manual operations to arrange the keying on each board. This is particularly troublesome when large numbers of circuit boards are involved. Others require high precision in the manufacture and assembly of the keying members in order to achieve a satisfactory arrangement having sufficiently large numbers of unique keys. The board and receptacle space consumed by keying arrangements is often excessive. Most arrangements are also unsatisfactory for retrofitting into existing systems that do not have a keying arrangement on their boards and receptacles.
The foregoing problems of the prior art are partially solved in an arrangement by T. G. Grau and A. M. Wittenberg for which U.S. patent application Ser. No. 10,306 has been filed contemporaneously with the filing of this application. In the Grau et al. arrangement, a first projection is located on a board in one of a plurality of first coded locations. These locations are arranged colinearly along an edge of the board associated with a guide rail of the receptacle in the direction of insertion of the board into the receptacle. A second projection is located on the board in an associated one of a like plurality of second coded locations arranged colinearly along the opposite edge of the board associated with another guide rail. Each of the first locations is associated with a different one of the second locations and the associated first and second locations are ordered in opposite directions. Each pair of associated first and second locations defines a unique key. First and second stops associated, respectively, with the first and second projections are located on the guide rails in the receptacle in the same spacial relationship as the projection.